Attempts to generate cytotoxic T-effector cell activity in vitro against measles virus infected autologous human peripheral blood leukocytes have been unsuccessful with the exception of cells from six volunteers. Cells from these donors have exhibited high levels of cytotoxicity which is both reproduceable and very specific. It could be demonstrated that T cells were required and sufficient to generate cytotoxic effector cells. Cold target inhibition studies confirmed the clonal, self-restricted nature of the response. Lack of sufficiently primed lymphocytes is the most likely explanation for the failure to generate cytotoxic effector cells for measles virus from many other donors. In certain donors a high amount of the activity (to which measles virus infected cells are reported to be particularly sensitive) seems to overshadow an HLA-restricted response.